1. Field of the Invention
The present invention relates to a magnetic memory device that is equipped with storage cells including magnetoresistive effect revealing bodies and is constructed so as to be capable of recording and reading out information.
2. Description of the Related Art
As one example of this type of magnetic memory device, the magnetic memory device disclosed by Japanese Laid-Open Patent Publication No. 2004-119638 proposed by the present applicant is known. This magnetic memory device is a magnetic random access memory (hereinafter, “MRAM”) and as shown in FIG. 5, includes a memory cell group 14 where a large number of storage cells 1 are disposed in the direction of word lines (i.e., in the X direction) and in the direction of bit lines (i.e., in the Y direction) to construct a matrix as a whole, a plurality of read word lines 6 that are disposed in the X direction and a plurality of read bit lines 5 composed of pairs of lines 5a and 5b that are disposed in the Y direction. The storage cells 1 each include a pair of storage elements 1a and 1b. The storage elements 1a and 1b include magnetoresistive effect revealing bodies 2a and 2b constructed using GMR (Giant Magneto-Resistive) or TMR (Tunneling Magneto-Resistive) effect, and two diodes Da and Db, that are connected to the magnetoresistive effect revealing bodies 2a and 2b, respectively in series and prevent backflow. The storage elements 1a and 1b are disposed at each intersection of the read word lines 6 and the read bit lines 5. In each storage cell 1 disposed at an intersection, one end of the magnetoresistive effect revealing body 2a is connected via the diode Da to the read bit line 5a out of the read bit lines 5 and the other end of the magnetoresistive effect revealing body 2a is connected to the read word line 6. In the same way, one end of the other magnetoresistive effect revealing body 2b in the storage cell 1 is connected via the diode Db to the other read bit line 5b that is paired with the read bit line 5a and the other end of the magnetoresistive effect revealing body 2b is connected to the common read word line 6. In each storage cell 1, one piece of information (digital information that is “0” or “1”) is stored based on whether the resistance of one out of the pair of magnetoresistive effect revealing bodies 2a and 2b is higher than the resistance of the other or lower than the resistance of the other.
Read circuits 23 (read circuits 23n and 23n+1 are shown in FIG. 5) for reading information stored in the storage cells 1 are connected to one end of each read bit line 5. Each read circuit 23 includes two switches (as one example, bipolar transistors Q1 and Q2 as semiconductor switches in FIG. 5) that are connected to the ends of the lines 5a and 5b that construct a read bit line 5, two sensing resistors R1 and R2 which each have one end connected to a power supply voltage Vcc and the other end connected via the bipolar transistors Q1 and Q2 to the lines 5a and 5b, and a circuit (for example, a differential amplifier circuit) 42 that reads and outputs information stored in a storage cell 1 by amplifying the difference between the voltages across the sensing resistors R1 and R2. On the other hand, constant current circuits 33 (constant current circuits 33m and 33m+1 are shown in FIG. 5) are disposed at one end of each read word line 6.
In this magnetic memory device, to read the information stored in a desired storage cell 1, the read circuit 23 corresponding to the read bit line 5 connected to the desired storage cell 1 is operated by applying a predetermined voltage from a Y direction address decoder circuit 22 to a bit decode line Y (for example, a bit decode line Yn) connected to that read circuit 23 to switch ON the transistors Q1 and Q2 of the read circuit 23. The constant current circuit 33 corresponding to the read word line 6 connected to the desired storage cell 1 is operated by applying a predetermined voltage to a word decode line (for example, a word decode line Xm) connected to the constant current circuit 33. In this state, the power supply voltage Vcc is applied via the sensing resistors R1 and R2 of the operated read circuit 23 to one end of the lines 5a and 5b that construct the read bit line 5 connected to the desired storage cell 1. On the other hand, one end of the read word line 6 connected to the desired storage cell 1 is connected to a voltage that is close to ground potential by the operated constant current circuit 33. By doing so, a current Ib1 flows through one magnetoresistive effect revealing body 2a in the desired storage cell 1 on a path composed of the sensing resistance R1, the transistor Q1, the diode Da, the magnetoresistive effect revealing body 2a, the read word line 6, and the constant current circuit 33 and a current Ib2 flows through the other magnetoresistive effect revealing body 2b in the desired storage cell 1 on a path composed of the sensing resistance R2, the transistor Q2, the diode Db, the magnetoresistive effect revealing body 2b, the read word line 6, and the constant current circuit 33.
Since the values of the currents Ib1 and Ib2 are controlled by the constant current circuit 33 so that the total of the currents is constant, based on whether the resistance of one of the magnetoresistive effect revealing bodies 2a and 2b is larger than the resistance of the other, one of the currents Ib1 and Ib2 will be larger than the other or smaller than the other. As the currents Ib1 and Ib2 change, the voltages across the sensing resistors R1 and R2 also change. Accordingly, by amplifying the difference in the voltages across the sensing resistors R1 and R2, the differential amplifier circuit 42 of the read circuit 23 reads and outputs the information stored in the desired storage cell 1.